The present invention relates to an arrangement for the hull of a vessel, wherein a skin plate is placed upon longitudinal stiffeners in a stiffening framework, said skin plate being built up as a composite member having an outer skin, a core and an inner skin, wherein the skin plate is designed to take external water pressure by making use of a diaphragm effect.
In this document, the term skin plate shall be understood to mean a plate area between two adjacent longitudinal stiffeners, and also a larger area composed of several skin plates of this kind which are connected to one another.
In a traditional hull, the construction of the plate and stiffener system is formed in such a way that the plates primarily bear the bending stress. The forces are fed from the plates over into the primary stiffeners (usually longitudinal stiffeners) and further over into the secondary stiffeners (usually the transverse stiffeners) and out into the side of the ship/longitudinal bulkhead in order to be distributed along the "ship's beam". In small vessels, the skin plate is often provided with a double-curved form. This contributes to outer pressure forces being taken up primarily as compressive stress (shell effect).
A hull built according to the traditional design can be optimalized with regard to weight or with regard to the cost of production. An optimal weight construction is characterized by relatively thin skin plates and a compact framework of primary and secondary stiffeners. This results in a complicated construction with high production costs. This complicated construction introduces several problems. In steel and aluminum hulls, as well as in glass fiber ones, a series of complicated connections between the different stiffening components is introduced. Cracking may easily occur here due to fatigue or delamination.
All the components of the hull are usually dimensioned so that the level of stress lies below a permitted elastic tension. When the plate sections are subjected to overloading, local deformation will occur at the points of attachment to the stiffeners and forces in the plates will gradually go from bending stress to tensile stress (diaphragm stress). This gives rise to lasting deformation (buckling) in metal hulls and local cracking in glass fiber hulls.
Constructions of hulls have been proposed with a view to making possible a reduction in weight and costs. Thus, for a metal hull, the use of a diaphragm effect is suggested in U.S. Pat. No. 4,638,754, wherein the plates are provided with a concave curvature when seen from the outside. Similarly, the use of a diaphragm has been suggested for skin plates built up as composite elements or so-called sandwich elements, having an outer skin, a core and an inner skin which are laminated together. In this connection, reference shall be made to International Patent Application No. PCT/NO90/00188 wherein skin plates are suggested which are concave when seen from the outside. The skin plates are built up as laminated elements and so-called diaphragm sections are obtained with the proposed concave form, i.e., concave plate sections which bear outer pressure with tensile stress. One disadvantage with this known construction is that it places demands on the geometry of the outer hull. In addition, with the manner of construction as described in PCT/NO90/00188 certain demands are made on the shearing strength in the core material.